1. Field of the Invention
The present invention relates generally to a cathode-ray tube with an electron gun assembly that emits at least one electron beam, and more particularly to a cathode-ray tube capable of improving focus characteristics of the electron beam and realizing a high resolution over the entirety of a screen.
2. Description of the Related Art
A currently predominant self-convergence in-line color cathode-ray tube includes an in-line electron gun assembly that emits three electron beams, which are horizontally arranged in line. The electron gun assembly includes an electron beam generating section that emits electron beams, a prefocus lens section that prefocuses the electron beams emitted by the electron beam generating section, and a main lens section that ultimately focuses the electron beams prefocused by the prefocus lens section onto a phosphor screen.
In general terms, the resolution of a color cathode-ray tube varies depending on the size and shape of a beam spot on the phosphor screen. In order to improve the resolution, it is thus necessary to form on the phosphor screen a beam spot having a minimum size and a circular shape with little deformation.
As a method of forming a small beam spot on the phosphor screen, it is effective to increase the lens power of the prefocus lens section and decrease the divergence angle of the electron beam that enters the main lens section. In order to increase the lens power of the prefocus lens section, it is necessary to make the potential of one of grids, which constitute the prefocus lens section, higher than the potential of the other grid, thereby providing a large potential difference between the grids of the prefocus lens section. However, from the standpoint of withstand voltage characteristics between neck pins, there are restrictions on the supply of high potential from the outside.
A method for solving this problem has been proposed, wherein a grid of the prefocus lens section is supplied with a high potential via a resistor that is disposed within the cathode-ray tube, thereby reducing the beam spot size. Jpn. Pat. Appln. KOKAI Publication No. 2000-331624, for instance, discloses an electron gun assembly comprising cathodes, a first grid, a second grid, a third grid, a fourth grid, a fifth grid, a sixth grid, a seventh grid, and a resistor disposed near these grids. The third grid is supplied with a voltage via the resistor, which is higher than a focus voltage and lower than an anode voltage. In this way, the third grid is supplied with a high potential within the cathode-ray tube. Hence, without degrading the withstand voltage characteristics between neck pins, the lens power of the prefocus lens section that is created by the second grid and third grid can be increased and the size of the beam spot formed on the phosphor screen can be reduced.
The electron gun assembly with the above-described structure, however, has the following problem.
In usual cases, a voltage of several-hundred V is applied to the second grid, and a voltage of 6 to 9 kV is applied to the third grid. The potential difference between the second grid and third grid is large. In order to further increase the lens power of the prefocus lens section, it is necessary to provide a still larger potential difference between the second grid and the third grid. This may lead to deterioration in withstand voltage characteristics between the grids that constitute the prefocus lens section. In addition, the cathodes are present near the prefocus lens section. The cathodes emit a great number of thermoelectrons, and produces a high-temperature environment. In consideration of the withstand voltage characteristics, a very undesirable environment does exit.
If a still higher potential than in usual cases is applied to the third grid in the electron gun assembly, the withstand voltage characteristics between the second grid and third grid deteriorate. Furthermore, owing to the rise in neck potential, the withstand voltage characteristics in the range between the cathodes and third grid also deteriorate. Consequently, the cathodes may be damaged due to discharge, or the resolution may be degraded by a voltage variation due to leak current. In worst cases, the cathode-ray tube may be destroyed due to discharge.